(Not Applicable)
(Not Applicable)
The present invention pertains generally to fluid flow control and, more particularly, to a piston positioning system and method for use thereof for positioning a piston within a cylinder of a pneumatic circuit. The piston positioning system includes a pneumatic valving device for manipulating a flow of pressurized pneumatic fluid within the pneumatic circuit.
Pneumatic systems typically involve a source of compressed air to provide a working pneumatic fluid. The compressed air is typically obtained from a compressor which is usually driven by an electric motor or an internal combustion engine. The compressed air is routed through pipes to control valves which selectively direct the routing of the compressed air. The control valves may be operated by electrically initiated solenoids or by pneumatic pilots. Pneumatic systems are typically employed to move an actuator which is conventionally comprised of a piston sealed within a cylinder. The piston may have a shaft extending out of the cylinder and connected to the component to be moved. The pneumatic system moves the piston by forcing air into a first end of the cylinder while simultaneously withdrawing or exhausting air out of a second end of the cylinder in order to advance the piston along the length of the cylinder. Conversely, the pneumatic system may force air into the second end of the cylinder while simultaneously exhausting air out of the first end of the cylinder in order to retract the piston in the opposite direction. By driving the air into alternate ends of the cylinder, the piston is moved such that the shaft can be displaced in any position for doing useful work.
Pneumatic and hydraulic systems are commonly used in large scale applications such as in power plants and refineries for controlling system components such as a working valve. In such applications, it may be desirable to quickly and repeatedly position the working valve to within thousandths of an inch. Such large scale applications involve extreme pressures on the working valve, necessitating very high volume flow rates of the pneumatic fluid into and out of the cylinder in order to re-position and maintain the piston location and ultimately the working valve position. Furthermore, the high volumetric flow rates occur at extreme working pressures in the working valve that must be reacted by the piston. The prior art discloses several actuators or piston positioning systems adaptable for use in large scale applications.
One such prior art device includes an actuator system which modulates a linear output shaft associated with a working control valve in response to a control signal input. The system includes a feedback control link, a pneumatically controlled hydraulic valving system and a hydraulic cylinder and piston controlled by the hydraulic valving system. The hydraulic valving system includes a three-position, four-way valve actuated by pneumatic binary output signals from a signal conditioner which is in turn controlled by the positioner. Hydraulic flow to the three-position, four-way valve may also be controlled from the signal conditioner in response to positioner output for effective actuation of the hydraulic piston and cylinder assembly. Although the system exhibits rapid response time and high accuracy in positioning the piston within the cylinder, the system is necessarily complex and costly in that it combines hydraulic circuit components with pneumatic circuit components. Furthermore, the reference device suffers from various other limitations such as safety risks associated with the flammability of hydraulic fluid and the dangers of high pressure hydraulic fluid lines. Finally, such a device suffers from a high risk of leakage due to the large number of joints connecting the many components to the piping.
Another prior art device employs a rotary servo valve coupled to a torque motor in a pressurized fluid system for positioning a piston within a cylinder. The torque motor controls the flow of fluid within the system by rotating the servo valve, the servo valve comprising a spool element within a sleeve assembly and having fluid passageways. The flow of fluid is adjusted in order to position a piston within a cylinder. In the event of a power failure, an arrangement of torque rods, springs and other mechanical elements are required to center the servo valve and halt the flow of fluid within the system. Furthermore, the torque motor is inherently inaccurate in its ability to position the servo valve and therefore precisely position the piston within the cylinder because torque motors have no detent or zero position. Torque motors instead require a mechanical brake mechanism to stop their rotation at the desired location. This mechanical brake mechanism must also be constantly applied in order to firmly maintain the piston position when the motor is not turning. Consequently, the torque motor must always be energized or actuated throughout operation of the servo valve system. The servo system therefore requires large amounts of power while the force acting against the motor remains present.
As can be seen, there exists a need in the art for a piston positioning system which utilizes an inherently safe working fluid. Also, there exists a need in the art for a piston positioning system that is of simple construction, of low cost and requires low maintenance. In addition, there exists a need in the art for a piston positioning system that is compact such that travel time and compressibility of the working fluid within the system is minimized in order to reduce the xe2x80x9cdead time on seatxe2x80x9d of a working valve. Furthermore, there exists a need in the art for a piston positioning system that can precisely and quickly position a working valve under extreme operating pressures. Finally, there exists a need in the art for a piston positioning system that can be autonomously and quickly neutralized in the event of a power failure or loss of working fluid pressure.
The present invention specifically addresses and alleviates the above referenced deficiencies associated with pneumatic actuator circuits. More particularly, the present invention is an improved piston positioning system for positioning a piston within a cylinder of a pneumatic circuit. The piston positioning system includes a pneumatic valving device for manipulating a flow of pressurized pneumatic fluid within the pneumatic circuit. As will be demonstrated below, the piston positioning system of the present invention differs from piston positioning systems of the prior art in that it utilizes a pneumatic valving device for manipulating a flow of pressurized pneumatic fluid within the pneumatic circuit.
In accordance with the present invention, there is provided a piston positioning system for positioning a piston within a cylinder of a pneumatic circuit. The piston positioning system is comprised of a controller, a pneumatic valving device, and a solenoid valve for collectively manipulating a flow of pressurized pneumatic fluid (e.g., air) within the pneumatic circuit. The pneumatic valving device is comprised of a reversible stepper motor, a servo valve, a four-way valve and a two-way valve, all of which are advantageously integrated into a single unit. In this regard, the pneumatic valving device replaces the assorted components that are typically networked together with a maze of pneumatic lines in conventional pneumatic actuation systems. In the present invention, a piston is sealed within a cylinder having first and second ends. The pneumatic valving device is actuated by energization of the four-way valve and the two-way valve through pilot lines. Feed lines then carry the flow of pneumatic fluid through the servo valve and into either the first or second ends of the cylinder. The stepper motor incrementally rotates and shifts the servo valve axially to locate the servo valve at a prescribed position. The pneumatic valving device therefore moves the piston by regulating the stepper motor and servo valve. The regulation of the servo valve alternately forces pneumatic fluid into the first and second ends of the cylinder while simultaneously exhausting pneumatic fluid out of the respective second and first ends in order to extend and retract the piston along the length of the cylinder.
Importantly, the piston positioning system of the present invention includes a fail safe mode. In the fail safe mode of operation, the solenoid valve may be autonomously closed in the event of a loss of electrical power or a loss of pneumatic fluid pressure within the pneumatic circuit. The closing of the solenoid valve acts to de-energize the four-way valve and the two-way valve. The four-way valve is de-energized due to the mechanical biasing force of the spring overcoming the reduced pneumatic pressure at the pilot passage. The two-way valve is de-energized due to the pneumatic fluid pressure within the servo valve overcoming the reduced pneumatic pressure acting at the pilot port. The de-energized four-way valve then effectively isolates the servo valve such that the flow of pneumatic fluid through the servo valve is blocked. The flow of pneumatic fluid is directed back through the four-way valve and into the second end of the cylinder. The de-energized two-way valve simultaneously opens and allows remaining pneumatic fluid to escape the first end of the cylinder through the servo valve such that the piston retracts towards the second end.